class Solution:
    def countLetters(self, s: str) -> int:
        n = len(s)
        i = ans = 0
        while i < n:
            j = i
            while j < n and s[j] == s[i]:
                j += 1
            ans += (1 + j - i) * (j - i) // 2
            i = j
        return ans
